Vacuum plasma generators can be an integral component of an alternating voltage gas discharge excitation arrangement that couples to a gas discharge device for treating a workpiece. Vacuum plasma generators can have different power classifications and different output signal forms.
In vacuum glass coating, for example, medium frequency (MF) vacuum plasma generators are used that have an MF output signal with power levels of between 30 and 300 kW. The MF output signal is mostly a sinusoidal signal having frequencies of between 10 kHz and 200 kHz. The output voltages may be from several 100 V to above 1000 V. In order to ignite plasma in the gas discharge device, the output voltages of vacuum plasma generators are often much higher than during normal operation.
In the plasma, brief and also longer-lasting spark-overs, so-called arcs, may occur, and such arcs are undesirable. An arc is generally identified by a break-up or a drop in the voltage at the vacuum plasma generator and an increase in the current at the vacuum plasma generator, for example, at the output of the vacuum plasma generator or at another location in the vacuum plasma generator. If an arc of this type is identified, it is extinguished or prevented from reaching a maximum level.
For example, in DE 41 27 505 C2, the output of the alternating current generator is caused to short circuit when an arc is identified, which is a suitable method for low-power MF generators. The higher the power levels become, the higher the voltages and currents also become and a switch for short circuiting would have to be able to short circuit such higher currents. Components of such a switch are usually larger and more expensive to fabricate. In some cases, components of a short circuiting switch would be connected in series and/or in parallel in order to be able to switch off the high voltages and currents. When arcs occur, the MF generator should supply as little residual energy as possible into the gas discharge device. For example, in MF generators for Flat Panel Display (FPD) production, an arc may lead to pixel errors, and a single pixel error can have a significant influence on the quality of a large surface-area (for example, for 19″ thin film transistor monitors) and consequently can cause a comparatively high level of damage.
In some designs, when an arc is identified, control to the vacuum plasma generator or to parts of the vacuum plasma generator is switched off so that no further energy flows into an output oscillating circuit of the generator. This procedure may not be sufficient for FPD production since there can still be too much residual energy in the output oscillating circuit of the generator, in the inductors of an output transformer that may be provided in the generator, and in the supply lines to the generator.